I'm just writing to confirm that you've received (title.) Our records show we sent you the manuscript on (month date) 2011.Editors prioritize how they read just like everyone else.
They read stuff they think will go fast first (generally); they've learned the hard way not to dawdle on projects from agents that go to auction often (Hello Brooks Sherman!)
They read things from their own writers more quickly than submissions as well.
That means if you have an agent the editor doesn't know well, or hasn't worked with, the agent MUST FOLLOW UP in a timely fashion or the submission sinks to the bottom of (I'm not kidding here) 11,206 emails.
7-30 days for material sent to an editor the agent has a longstanding relationship with.
30-60 days for everyone else.
How can you tell how often your agent follows up on submissions? Ask.
Your agent should be able to send you a submissions data base with the name of the editor, the publishers, when the ms was sent, and when s/he followed up. If s/he doesn't list dates: ASK.
The worst thing in the world is to think no one likes your work when the truth is simply no one has read it.